Wave translation system



y 9, 1935; H. s, BLACK- 2,007,172

WAVE TRANSLATION SYSTEM Filed March 29, 1955 V FIG. GRID OF FIRST 572aPl/WE 0F LAST sue/=- FIG-2 INVENTOR I H.$. BLACK A TTORME Y PatentedJuly 9,1935

PATENT OFFICE,

WAVE TRANSLATION SYSTEM Harold S. Black, New York, N. Y., assignor toBell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application March 29, 1933,Serial No. 663,316

8 Claims. (01. 179-171) This'application is a continuation in part of mycopending application, Serial No. 606,871, filed April 22, .1932, forWave translation systems.

This invention relates to wave translation systems, as for example,electric wave amplifying 1 systems.

Objects of theinvention are stabilization, distortion suppression andimpedance control in such systems.

In accordance with one feature of the invention, in a push-pull orbalanced system in which balance suppresses distortion to a degreedependent upon the degree of balance attained, the degree of balance isincreased by retroaction or feedback in the system, for example, asexplained hereinafter. e

.In accordance with another feature of the invention, reduction ofdistortion is achieved by choosing for a push-pull or balanced system aload impedance which'causes objectionable modulation produced in thesystem to be less than if the load impedance had the value of the outputimpedance which it would face in the absence of feedback,'and then, asindicated hereinafter, for example, causing feedback to match the outputimpedance-to the chosen load impedance in order to prevent objectionablewave reflection, which might result in objectionable crosstalk, forinstance. (For any given frequency the impedance of a network betweenany two points is con-- sidered the ratio that a voltage applied acrossthe points from an external network would hear to the internetworkcurrent.)

Also, in accordance with the invention, the above mentioned features maybe combined.

In one specific aspect, the invention is an amplifier with negativefeedback of fundamental and distortion waves so as to increase stabilityof operationand reduce distortion production as-explained in the abovementioned copending application, the. amplifier being of the balanced orpush-pull type in accordance with the present invention. It has beenfound that the distortion suppression attainable in such a negativefeedback amplifier exceeds the sum of that attainable by the balancingaction without feedback and that.

attainable by the feedback action without balance; feedback can improvethe balance or make the two sides of the push-pull system more nearlyalike as regards their transmission properties. In general, attainmentof this improvement in balance. is facilitated by making the two sidesof the push-pull amplifier independent as regards feed-' back operationor-in other words by making each because, as explained hereinafter, the,

side an individual feedback amplifier so that neither side of thepush-pull system will materially affect the feedback operation of theother. side, and is also facilitated by preventing reaction of the inputor output connecting circuit 5 for either side of the push-pull circuitupgn the feedback operation of that side. To indicate in a simple waywhy this is so, and how the negative feedback action can make amplifiersmore nearly alike as regards their transmission properties, certainaspects of operation of feed-back amplifiers will now be pointed out,after the significance with which this specification uses certain termsand symbols relative to operation of vacuum tube amplifiers has beenindicated.

Amplification of an amplifier without feedback is what the voltage onthe grid of the first tube must be multiplied by to obtain the phase andmagnitude of the resulting voltage generated in the plate circuit-of thelast tube, or the voltage of an equivalent fictitious generator inseries with the internal plate resistance of the last tube. Thisamplification will be designated as ,u (and is a complex quantity).Amplification ratio is the absolute-value of the amplification. By theterm gain as used in this specification is meant voltage'gain, i. e.,twenty times the logarithm of the amplification ratio.

The complex quantity #5 will be used herein to designate the ratio bywhich a voltage of a wave is modified in a single propagation around theclosed feedback loop of a feedback amplifier. It follows that p is thecomplex quantity by which a driving voltage in the space path of thelast tube, in series with the internal plate-filament impedance R0 ofthat tube, must be multipliedto give the voltage that it--the drivingvoltage alone-acting through the feedback path, will produce on the gridof the first tube.

As shown in the above mentioned copending application, the amplificationof a feedback am plifieris and the corresponding change in amplificationcaused by the feedback action is The quantity 50 back is describedaspositive feedback or negative feedback according as the absolute valueof 1 1 is greater or less than unity.

As pointed out in that application, when afl 1 the amplification withfeedback approaches are made equal, and therefore, the suppression;

by balance, of the even order modulation products generated in the last,stage is greatly facilitated and the usual difiiculties of. obtainingsufficiently close matching of tubes, interstage networks, etc., aregreatly reduced. Moreover, this equality of gains and phase shifts isnot affected by tube changes, changes in direct current plate and gridvoltages, etc., but is constant or stable with time.

In one specific embodiment of the invention disclosed hereinafter, withthe amplification in each of the two amplifiers in the two sides of thepush-pull circuit substantially equal to the angle of p is madesubstantially zero, and

- plifier approaches consequently the phase shift through each am-However, as indicated hereinafter, the invention is not limited tohaving the angle of 5 substantially zero, nor to having pfi 1.

In an amplifier, ordinarily negative feedback action (apart frombalance) reduces distortion products of odd and even ordersapproximately equally, whereas balance by push-pull action (apart fromthe feedback action) reduces the even order products primarily.Therefore, since the second order products are usually very much greaterthan the modulation products of higher orders, it is especiallyfortunate that by employing both negative feedback and balance, thesecond order products can be reduced not only directly by feedbackaction, but also by balance and further by improvement of balanceproduced by the negative feedback.

In accordance with one feature of the invention, each side of thepush-pull circuit can be prevented from unduly effecting the feedbackoperation of the other side, for example, by employing separate outputbridge networks for the two sides (i. e., employing output bridgesindividual to the two sides), and feeding backfrom the output bridgenetwork on each side of the circuit to the input of that side of thecircuit, with each output bridge network giving conjugacy between thefeedback path for its side of the circuit and the output connectingcircuit for that side of the circuit, for giving any desired closenessof approach to such conjugacy. If de sired, in accordance with theinvention, an input bridge, also, can be used in each side of thepushpull amplifier, each input bridge giving conjugacy \(or anydesiredcloseness of approach to conjugacy) between the feedback path forits side of the push-pull circuit and the input connecting circuit forthat side of'the push-pull'circuit,

' so that theinput connecting. circuits can not unduly afiect thefeedback operation of either side of the circuit or in other words cannot affeet the pin either side of the circuit.

However, the invention is not limited to the' use of the input bridgesor the output bridges.

In one specific aspect the invention is a pushpull amplifier withnegative feedbackimproving the balance as described above and with aload impedance substantially greater than the 'output'impedance that theload impedance would face if there were nofeedback, and with thefeedback raising the output impedance to a value that matches the loadimpedance. (The amplifier output impedance without feedback is the valuethat the amplifier output impedance would have if a could in some way bereduced tozero and, at the same time, the remaining properties of the,ufi-path or closed feedback loop or system not be altered.)Objectionable modulation produced by the amplifier is reduced by thusworking the amplifier into a load impedance larger 7 than the value thatthe amplifier output impedance would'have in the absence of feedback;yet, since the feedback action matches the output impedance to thisincreased load impedance, objectionable wave reflection is obviated.Such reflection might, for example, result in objectionable cross-talk,for instance, in the case of a string or single line vacuum tubeamplifiers A1 and A2, which are alike, connected back to back inpush-pull relation. Amplifier A1 comprises a vacuum tube or any numberof vacuum tube stages T1 in tandem; and amplifier A1 comprises a tube orstages T2 like the tube or stages of am- The tubes T1 and T2areconnected to the outgoing circuit 2 throughtwo output bridge net-'works 5 and 5 and output transformer I. The transformer has two primarywindings 8 and 8. The winding 8 forms one diagonal of the bridge circuit5; and thewinding 8', forms one diagonal of the bridge circuit 5'. Thefour ratio arms of the bridge circuit 5 'comprise four resistances orirnpedances R0, KRo, KR and R, respectively, the

impedance Rn being the plate-cathode impedance in the last of the tubesor stages T1. The four ratio arms of bridge circuit 5' likewise comprisefour resistances or impedances R0, KRo, KR and R, of which the impedanceR0 is the platecathode impedance in the last of the tubes or stages T2.4

The incoming circuit l is connected to the tubes T1 and T2 through inputtransformer 10' and two input bridge networks II and II. The transformerIII has two secondary windings l2 and I2. The winding l2 forms onediagonal of the bridge circuit II; and the winding I2 forms one diagonalof the bridge circuit II. The four ratio arms of the bridge circuit Hcomprise resistances r1, kn, kr and 1, respectively, the grid andcathode of the first of the tubes T1 being connected across arm n. Thefour ratio arms of bridge circuit H likewise comprise four resistances11, km, hr and r, the grid andeathode of the first of the tubes T2 beingconnected across this resistance r1. Negative feedback in amplifier A1is provided through a feedback path F comprising conductors l6 and I1,and, as explained in the above mentioned copending application, rendersthe operation of the amplifier more stable than it would be withoutfeedback Cal and reduces distortion produced by the amplifier.Similarly, negative feedback in amplifier A2 is provided through afeedback path F comprising conductors I6 and I1, and increases thestability of 'operationof the amplifier and reduces its distortion. Asindicated above, the feedback action improves the balance of the twoamplifiers for second order modulation components, rendering theirfundamental driving voltages in the last stage more nearly equal andopposite. Over the utilized frequency range, each of the amplifiers mayhave ;3 1 so that the amplification for each is approximately and B maybe the same constant quantity for each-amplifier. As noted above, withthe phase angle of 15* equal to zero, the phase shift through eachamplifier (from the first grid to the lastspace path driving voltage)approaches 180.

As described above, the windings 8 and I2 are in diagonals of bridges 5and II, respectively. The other diagonals of these bridges comprise thefeedbackpath F, so. the path F is rendered conjugate to the windings 8and I2 by the bridges 5 and II, respectively. Similarly, path F' isrendered conjugate to windings 8 and I2 by bridges 5' and II,respectively. Thus, neither of the amplifiers A1 and A2 can affect theoperation of the other, (i. e. neither amplifier can act as atransducer, either active or passive, affecting the value of B for theother amplifier), nor can any of the input or output transformers orconnecting circuits for either amplifier affect B or the feedbackoperation of either amplifier. In the circuit, as shown, when thebridges are balanced and their ratio arms are resistances, the angle ofp is substantially zero. The output bridges, especially, areadvantageous in preventing the feedback in either of the amplifiers A1and A2 from being affected by the outputfrom the other amplifier.However, if desired either the input bridges or the output bridges, orboth, can be omitted; and moreover, with or without the omission ofbridges, the feed-back for the pushpull amplifier can be effectedthrough the input or output transformers, or both, so as to reduce thedistortion introduced in the transmitted waves by the transformer aspointed out in the abovementioned copending application, or can beeffected without feeding back by primaryto-secondary transmissionthrough the transformers.

Only the alternating current circuits of the amplifier A are shown inFig. 1, the electromotive force sources, choke, coils, stoppingcondensers,

etc., required for energizing the amplifier or con-.

57,, 65 or 66.of the.above mentioned copending application.

Fig. 2 shows a two-stage push-pull amplifier without input or outputbridges suchas those of The amplifier of Fig. 2 may besuitable Fig. 1.for amplifying frequency waves of a wide frequency range, as forexample, for use in repeaters of an open wire multiplexcarrier'telephone system of the general type of the present commercialtype C carrier telephone system describe-d by Messrs. H. A. Afiel, C. S.Demarest and C. W. Green in the Bell System Technical JournalQJuly 1928,pages 564 to 629. In such cases it is important, especially from thepoint of view of reducing reflection or suppressing crosstalk, that theamplifier output impedance match theload impedance. (The load impedancemay here be considered as including the impedance .of the amplifier.output transformer; and the amplifier output impedance may be consideredas the imimpedance that is to match (i. e. equal) the amplifier outputimpedance becomes correspondingly greater and, consequently,objectionable modulation products are reduced.

In the push-pull amplifier of Fig. 2, the first stage comprises twotubes 23 and 23 in push-pull relation. The second stage comprises twotubes 24 in parallel and two tubes 24' in parallel, the tubes 24 beingconnected in push-pull relation with thetubes 24'. Each of these sixtubes may be, for example, Western Electric 101-F type tubes. Aninterstage transformer 30 connects the two stages. It has two secondarywindings 32 and 32'. Winding .32 is connected in the input circuit oftubes 24, in series with a 450-ohm feedback resistance 25 which iscommon to their input circuit and their output circuit. Similarly,winding 32' is connected in the input circuit of tubes 24', in serieswith a, 450-ohm resistance 25' which iscommon to their input circuit andtheir output circuit. Thus, the second stage is a pushpull, negativefeedback amplifier A comprising two separate single string or singleline negative feedback amplifiers A'i and A2, the amplifier A'icomprising tubes 24 in parallel, and the amplifier A2 comprising tubes24 in parallel. Batteries 0 are grid biasing batteries for the tubes 24and separating the alternating current output cir-- cuits of tubes 24and 24' from the source of unidirectional plate current for those tubes.With respect to the circuit of Fig. 2 and also the circuit of Fig.1,thmdesignation of amplifiers on opposite sides of a push-pull circuit asseparate has refer ence to their transmission circuit, and not to the.circuits for energizing the amplifiers or conditioning them foroperation.

Each of the amplifiers A'1 and A'2 in Fig. 2,

.having a feedback impedance 25 or 25' common to the input and outputcircuits of the amplifier, is a negative feedback amplifier circuit ofthe type referred to in the above mentioned copending application asthecommon impedance type,

A retard coil cated in the description of that figure, the negativefeedback action increases the amplifier outshown in Fig. 34 of thatapplication. As indiput impedance,- (from Z0, its value withoutfeedback), by the factor [1-(pfi) z]. The expression ([Lfi)z designatesthe value that s would have if the load impedance were zero; and

. grid-cathode impedance, which faces the incom- .ing circuit (winding32 or 32 in Fig. 2) and the feedback impedance in series; and C is theimpedance of the incoming circuit. Thus, in

. Fig. 2 the negative feedback action can be caused to increase theoutput impedance of amplifier A'i and also that of amplifier A: to avalue, for example, approximately twice-R0, i. e. twice theplate-cathode impedance in tube 24 or tube 24?. The transformer 1 thencan be given the proper impedance transformation ratio to match theimpedances on the two sides of the transformer, so that circuit 2 willbe terminated in its own impedance at the amplifier and-reflection,consequently, will be obviated and at the same time the transformer willpresent to the tubes twice their own impedance.

The 101-F type of tube, when worked into twice its own impedance, has abetter modulation characteristic (i; e. has a lower modulation poweroutput fora given fundamental power output) than when worked into amatched impedance; and the circuit of- Fig. 2 combines (1) thisimprovement in linearity due to working the outputtubes into. twicetheir impedance with (2) the improvement in linearity that the negativefeedback action produces by reducing the ratio of modulation voltage tofundamental voltage generated in the tubes 24 and 24' (3) theimprovement in stability that the negative feedback action produces inthe operation of each of the amplifiers A1 and A's, (4.) the improvementin linearity that results from merely pushpull operation, and (5) theimprovement in linearity that results from the improvement that thenegative feedback produces in the balance of the two sides of thepush-pull circuit, (as explained above in connection with Fig. 1). Thus,modulation is reduced in four ways simultaneously, the reductionproduced directly by push-pull action and that-produced directly byfeedback action combining and being augmented by that due to theincreased load impedance made feasible by feedback and that due to theimprovement in balance effected by feedback.

Since the amount-by which negative feedback action in amplifier Areduces the amplifier gain depends on the amount of the negativefeedback,

the gain reduction may be made small by having the amount of feedbackvsmall. Moreover, since the load impedance has been increased (byincrease in the ratio of transformer 1) to match the amplifier outputimpedance as increased by the negative feedback, the alternating platecurrent of the output stage has thereby been decreased for a given poweroutput to the load and consequently the electron emission from thecathodes of the last stage need not be as great as in the case of thelower load impedance. Therefore, in the case of the increased loadimpedance, tubes may be used in the last stage which require lessheating energy than the tubes that would be required in-tlie case of thelower load impedance; and this not only effects important economy inpower supply but facilities using tubes of higher amplificationfactorjto compensate for the effect of the negative feedback in reducingthe amplifier gain, so that the amplifier gain may, for example, beapproximately the same as it would be if the tubes of loweramplification factor were employed and no feedback were employed. Thefeedback tends to reduce the amplifier gain; but this tendency can becounteracted by increasing the amplification factor of the tubes,(since.afl 1), so that while the amplifier. output impedance is changednot only by the use of tubes of higher amplification factor or plate.resistance, but moreover, by the feedback action; nevertheless, theamplifier gain remains substantially unaltered. It has been found thatin the system of Fig. 2 employing the negative feedback and usingthe101-F type tubes mentioned above, which have a normal filamentcurrent of approximately one half ampere, the gain and the power outputcapacity of the system may be substantially as great as for the systemwithout the feedback but with tubes (two Western Electric 101-D orL-type tubes in the first stage and four Western Electric 104-D orO-typetubes in the last stage) having a normal filament current .ofapproximately one ampere, and that the modulation produced by the systemis reduced sufiiciently to render unnecessary tube selection forpush-pull balance, which was a practical necessity without the feedback-It is noted that the output transformer in Fig. 2 may step the amplifieroutput impedance down to the impedance of circuit 2, the negativefeedback action in the amplifier being used to increase the amplifieroutput impedance from a value greater'than the impedance of circuit 2 toa value still greater.

As indicated above, the invention is not limited to having ,ufi 1. It iswithin the scope of the invention to have p l in the circuit of Fig. 2as well as in the circuit of Fig. 1. doing so in the circuit shown inFig. 2 would tend to increase the degree in which the feedback raisesthe output impedance).

Moreover, control of impedance by feedback, as contemplated for theinvention, is not limited to raising impedance. For instance, in anegative feedback amplifier circuit in which the voltage to be fed backis derived from a voltage across the load, the negative feedback canlower the amplifier output impedance, as pointed out, for instance, inconnection with Fig. 31 of the above mentioned copending applicationshowing such a circuit, and in accordance with the present inventionsuch negative feedback, for example, can be used to lower the outputimpedance of a pushpull amplifier and at the same time to match theoutput impedance and the load impedance (for example, the impedance ofthe high impedance winding of a step-down transformer facing theamplifier output impedance and forming part of the load). This may bedesired, for instance, in case the tubes of the last stage of theamplifier are pentodes, it being customary (without feedback) to worksuch tubes into approximately one fifth. of their impedance rather thaninto a matching impedance, in order to improve their operation, as forexample, in order to reduce objectionable modulation that they produce.

While in accordance with the present inven- (Ordinarily,

tion, impedance is controlled by feedback inspeciflc types of circuitsor systems, the above mentioned copending appplication and my copendin'gapplication Serial No. 663,317, filed of even date herewith, for Wavetranslation systems,'present .dominating claims not limited to thosespecific Q the two sides of the push-pull circuit, respectivetypes ofcircuits or systems. ,What is claimed is:

1. A negative feedback, push-pull amplifier comprising two feedbackpaths for the two sides ,of the push-pull circuit, respectively, twooutgoing circuits for the two sides of the push-pull circuit,respectively, and two output bridges for ly, each bridge connecting inconjugate relation one of said feed-back paths and one of said out- 7going circuits.

2. A negative feedback, push-pull amplifier comprising two feedbackpaths for the two sides 'of the push-pull circuit,'respectively,. twoincoming circuits for the two sides of the push-pull circuit,respectively, and two input bridges for the two sides of the circuit,respectively, each bridge,

plifier having a load impedance of value that causes objectionablemodulation produced in the amplifier to be less than if the loadimpedance had the value that the amplifier output impedance would havein the absence of feedback in the amplifier, and means for producing inthe amplifier negative feedback that improves the pushpull balance ofthe amplifier and matches the amplifier output impedance to said firstmentioned load impedance. I

' 5. A wave translating system comprising an amplifying valve and a loadtherefor having an impedance that-causes objectionable modulation.-

produced in the system to be less than if the load impedance had thevalue that the output impedance of the valve would have in the absenceof feedback, and means for producing in said systern negative feedbackthat matches the output impedance of the valve to said first mentionedstructure'giving it transmission characteristics, with said feedback,approximately the same as those of a corresponding valve of the samepower capacity without feedback.

' 6. A wave translating system comprising a vacuum tubeamplifying'device, means producing negative feedback in said device, anda load for said device, said device having its feedback, its loadimpedance and its amplification factor such that the amplification ofsaid device is substantially the same and its ratio of outputfundamental to output of modulation is at least substantially as greatas if the feedback were absent and said device, were replaced by one ofthe same type, but with lower amplification factor and with powercapacity in the absence of feedback equal to the power capacity of thefirst men tioned device with its feedback.

7. A wave'translating system comprising two negative feedbackamplifiers, each having #5 of greater order of magnitude than unity, anincomingcircuit, an outgoing circuit, and means connecting saidamplifiers in push-pull relation between said incoming and outgoingcircuits, said means comprising a pair of input circuits and a pair ofoutput circuits for said amplifiers and means opp itelyassociating saidinput circuits with said incoming circuit and oppositely associatingsaid output circuits with said outgoing circuit. v i

8. In combination, a negative feedback amplifier comprising a feedbackpath individual thereto' rendering feedback therein negative, a, secondnegative feedback amplifiercomprising a feedback path individual theretorendering feedcuits for said amplifiers, said amplifiers beingsymmetrically and oppositely disposed with respect to each of saidcircuits.

HAROLD S. BLACK.

, load impedance, said valve having electrode.

'back therein negative, and input and output cir-

